Hydrogen is an important feedstock for many chemical and petrochemical processes. However, hydrogen production is associated with large amounts of carbon dioxide (CO2) emissions. Accordingly, it is desirable to not only provide a means to produce hydrogen but also a means to recover the carbon dioxide associated with the hydrogen production. With increased emission regulations and a possible future CO2 tax there is a need to develop carbon dioxide capture solutions. The cost of capture can impact the cost of hydrogen production. Therefore, it is important to develop a solution with lower cost of capture and improved efficiency of hydrogen production plant.
Physical or chemical solvents such as rectisol, selexol, amines, potassium carbonate etc. have been used traditionally for many decades to absorb carbon dioxide from syngas in hydrogen plants, HYCO (hydrogen and carbon monoxide co-production) plants. However, the process of solvent absorption requires absorber and stripper columns with very high capital costs. The scrubbing solvent needs to be regenerated by temperature swing or pressure swing in order to release the absorbed carbon dioxide. The regeneration process can involve large amounts of steam or compression energy resulting in high operating costs. Another disadvantage of using the solvent absorber is that the purity of the recovered carbon dioxide may not be very high and further processing using liquefaction and partial condensation may be needed. Example U.S. Pat. No. 6,500,241 describes the use of acid gas removal unit and auto refrigeration unit for removing carbon dioxide from syngas and PSA off-gas. U.S. Pat. Nos. 4,553,981 and 7,682,597 describe the use of a carbon dioxide scrubber downstream of the shift reactor to remove carbon dioxide from syngas.
Carbon dioxide can be captured from a hydrogen plant by using cryogenic processing viz, partial liquefaction or distillation at low temperatures. Such a process is favorable at higher carbon dioxide concentrations. In hydrogen plants, carbon dioxide can be captured from several different locations in the process train including process syngas or flue gas. Flue gas processing can pose several challenges because of many new impurities that therefore make the capture process very expensive. Carbon dioxide from process gas can be captured from high pressure syngas before pressure swing adsorption unit or from pressure swing adsorption off-gas. The concentration of carbon dioxide in the gas before pressure swing adsorption is much lower than in the pressure swing adsorption off-gas and hence the pressure swing adsorption off-gas is more suitable for cryogenic separation. U.S. Pat. No. 6,301,927 provides an example of an auto refrigeration process that employs the use of a compression and expansion turbine to liquefy carbon dioxide and further separate it from other gases. Another patent, FR Patent No. 2877939 provides a way to remove carbon dioxide from pressure swing adsorption off-gas by using successive steps of compression and cooling to remove carbon dioxide by partial liquefaction and/or distillation. This patent describes the use of a membrane on the non-condensable gas to permeate hydrogen and recycle hydrogen back to the pressure swing adsorption unit in order to increase hydrogen recovery. However, carbon dioxide recovery for the unit is not very high. In U.S. Pat. No. 4,639,257, provides the use of carbon dioxide selective membrane in combination with distillation column for a carbon dioxide containing gas mixture in order to increase the recovery of carbon dioxide. A carbon dioxide selective membrane is used on the overhead of the distillation column with carbon dioxide rich permeate recycled back to feed or to the distillation column itself. Another carbon dioxide selective membrane is proposed for the feed gas before the distillation column in case the concentration of carbon dioxide is below equilibrium concentration at the freezing temperature of the mixture. However, this patent is suitable for a gas mixture containing carbon dioxide, nitrogen, methane and hydrocarbon. U.S. Patent Publication No. 2010/0129284, describes the use of a hydrogen selective membrane, a carbon dioxide selective membrane in combination with carbon dioxide liquefier in order to increase the recovery of hydrogen and carbon dioxide. However, carbon dioxide selective membrane is always located upstream of the liquefier requiring additional compression of the carbon dioxide permeate from the membrane feeding to the liquefier.
Hydrogen plants can emit large quantities of carbon dioxide into the atmosphere. Carbon dioxide capture solutions have been proposed in the past using several different separation techniques like absorption, cryogenic, adsorption or membrane. There is always some hydrogen loss from pressure swing adsorption processes. In addition, there is some carbon dioxide loss from the capture process which can be recovered by improving the carbon dioxide capture process. If additional hydrogen and carbon dioxide can be recovered from the capture process there can be significant savings with regards to the size of reformer, natural gas consumption, carbon dioxide tax etc. for the same size hydrogen plant.
Accordingly, there still exists a need for a process to recover both hydrogen and carbon dioxide with a carbon dioxide recovery rate of at least 90% from syngas, as well as a process for producing hydrogen in a hydrogen generation plant that allows for the overall capture of at least 50% of the carbon dioxide, preferably at least 90% of the carbon dioxide, in the hydrogen production process.